Rotary string trimmers utilizing lengths of nylon monofilament line, or the like, are widely used for cutting weeds and other vegetation. Typically, the trimmer line which is used in such machines is extruded nylon monofilament, generally having a circular cross section. The cross-sectional diameter generally varies from 0.0050xe2x80x3 for non-commercial or home use up to 0.155xe2x80x3 for commercial use or heavy duty brush cutting. Typical rotational cutting speeds vary from approximately 2000 RPM (for large diameter line) to 20,000 RPM for smaller diameter line. At these high speeds of rotation, substantial wind noise is produced by the line, as well as significant drag. The drag on the engine causes greater fuel consumption for gasoline powered string trimmers, and results in higher electrical use for electric motor driven string trimmers.
Efforts have been made to provide discontinuities or distortions on the surface of string trimmer line; and such distortions have been found to reduce noise to some extent. The United States patent to Mize et al. U.S. Pat. No. 4,186,239 discloses a cutting filament made of nylon with a plurality of transverse notches on its surface. The notches are provided to prevent the nylon filament from fraying and fibrillating on the ends as it is used. The notches tend to define predetermined breaking edges at the free end of the cutting element.
The U.S. patents to Harbeke U.S. Pat. No. 5,220,774 and Warthen U.S. Pat. No. 3,063,094 both are directed to filaments which include spiral discontinuities extending along the length of the filaments. These discontinuities may be in the form of spiral grooves (as shown by both Warthen and Harbeke), or they may be in the form of a helical spiral protrusion, as disclosed in the Harbeke patent. As noted in the Harbeke patent, the utilization of these elongated spirals or grooves is believed to shed vortices in a non-periodic manner to reduce noise produced by the rotating filament. It also should be noted that if the helical discontinuity is formed by means of a spiral groove along the length of the filament, the cross-sectional area of the filament is reduced by the depth of the channel or discontinuity which is formed. On the other hand, if the discontinuity is an additional spiral rib on the surface of the string trimmer line, the overall diameter of the holes through which the line must pass on machines with which it is used needs to be greater than for conventional string trimmer line of comparable bulk.
The United States patent to Morabit et al. U.S. Pat. No. 5,761,816 is directed to the formation of an aerodynamic profile for the cross section of a string trimmer line. Many of the examples which are shown in this patent are somewhat complex in shape. The intent of these highly specialized cross-sectional shapes of string trimmer line, however, is to reduce the coefficient of drag as the line spins about the rotating hub of the trimmer machine when it is used.
The U.S. patent to Behrendt U.S. Pat. No. 5,687,482 is directed to a rotating trimming line which is stated to have reduced noise. The trimmer line disclosed in this patent has a square or rectangular cross section; and the line is twisted about its central axis in a continuous spiral. Thus, the four edges extend along the line in a helical configuration. Behrendt states that this twisting of the line causes the flow resistance to be diminished, the noise to be reduced, and the rotational speed of the tool to be increased, with the same performance. It further is stated in Behrendt that another advantage of the line is that its resistance to wear is not reduced by depressions or notches, such as is the case with rotation trimmer devices provided with grooves or stellate profiles.
The corner-to-corner distance across the square, the diagonal, can be no larger than opening utilized for standard circular lines (for example, 0.095xe2x80x3), in order for this line to be used with conventional string trimmer devices. The weight of the Behrendt line per unit of length for any given size then is less than conventional circular line. Thus, less bulk of line is available for impact in trimming vegetation; but the effectiveness of string trimmers is directly related to the mass of the line and the speed at which it is rotated.
It is desirable to provide a string trimmer line which produces low drag and reduced noise, which is simple and effective to manufacture, and which overcomes the disadvantages of the prior art devices.
It is an object of this invention to provide an improved string trimmer line.
It is another object of this invention to provide an improved low-noise string trimmer line.
It is an additional object of this invention to provide an improved low-noise string trimmer line which is simple to manufacture.
It is a further object of this invention to provide an improved low-noise string trimmer line having a generally oval shaped cross section, at least part of which is helically twisted about its longitudinal axis.
In accordance with a preferred embodiment of this invention, a string trimmer line consists of an elongated length of filament at least a portion of which has an oval cross section, with a generally centrally disposed longitudinal axis, and where at least a portion of the line is twisted about the centrally disposed axis.